1g space travel

[brötchen]

there is a similar thread already on this forum but my scenario is just a little bit different and i don't want to hijack threads...so.
imagine you have a space ship that accelerates in such a way that the passengers experience a constant acceleration off ~10m/s² (1g). now how long would it take this think to get to alpha centauri ? or the next galaxy. and more importantly : how do i account for relativity in the calculations? ( I would just calculate it my self if I would know how to do so..)
will relativity even be relevant? Hoe many Gs would we have to take to get to alpha centauri within a human life time?

[Madwolf]

Ive done alot of amatuer work with this actually, and even just accelerating at 1G for ~30 days on each end your looking at a one way trip of ~ 11 years. More like....5-8 under constant accel IIRC.

Yes, Relativity does matter. (Under the 11 year journey your velocity would be ~ 30% C) As for calculating it, just take the equations an integrate over time? Or find your average velocity...

[nbonaparte]

It's a simple calculation I won't do now, but eventually you're going to hit the speed of light and have to maintain a constant (or close to it) speed. Your acceleration should decrease asymptotically.

Alright, on second thought, I will calculate it. Accelerating at 10 m/s^2 will reach the speed of light in a little under a year. That's 2 years for acceleration/deceleration. Alpha Centauri is 4.37 ly away from Earth. This is an approximation, but I would think that gives you 1 year of cruise time. It can be done easily within a human lifetime. You could even get back. Time dilation makes it even shorter for the traveler. That means you could get a lot further than Alpha Centauri in the traveler's lifetime. However, your friends on Earth would probably be dead on your return.

[kromagnon]

If you want some real numerical answers, you'll have to give an upper limit on the speed. You can't reach light speed, and peaking at .99c is a lot different than peaking at .9999c

[EricH]

The OP question actually appears to be, "If I set the apparent acceleration in the ship frame to be 10 m/s² throughout the journey (with negligible time to reverse the thrust vector at the halfway point), what would be the travel times to Alpha Centauri, in both the accelerating and rest frames?" So, assume your star drive can produce whatever finite amount of force is necessary, without worrying about where the energy comes from.
The last two commenters apparently didn't make the underlined assumption, but it's important; you don't want to crush the passengers into jelly, just so that observers outside the ship will perceive its acceleration as 1 g.

[gmalivuk]

Yeah, disregard the posts about hitting the speed of light. It seems pretty clear to me as well that the OP is talking about a constant *apparent* acceleration in the ship's frame.

For which I recommend http://www.weburbia.com/physics/rocket.html to get the equations. Conveniently, 1g works out to about 1.03 ly/y^2, so you can run the equations with years of time and lightyears of distance and an acceleration of 1.03 and get the answers you want.

[kromagnon]

The OP question actually appears to be, "If I set the apparent acceleration in the ship frame to be 10 m/s² throughout the journey (with negligible time to reverse the thrust vector at the halfway point), what would be the travel times to Alpha Centauri, in both the accelerating and rest frames?" So, assume your star drive can produce whatever finite amount of force is necessary, without worrying about where the energy comes from.
The last two commenters apparently didn't make the underlined assumption, but it's important; you don't want to crush the passengers into jelly, just so that observers outside the ship will perceive its acceleration as 1 g.

Good catch, I did miss that.

I have the urge to write a program to solve this....

Code: Select all

import relativity
(insert python here)


[nbonaparte]

please, please tell me that's a real module.

[Goemon]

If you mean "please tell me those equations are correct" - then ok. They are.

But of course it's pretty hard to maintain constant acceleration of 1g for an extended period of time for a number of reasons

[polymer]

If you mean "please tell me those equations are correct" - then ok. They are.

But of course it's pretty hard to maintain constant acceleration of 1g for an extended period of time for a number of reasons

Is it realistically possible though <_<

[gmalivuk]

How realistic it is depends on how long you intend to keep it up. As time goes on, the likelihood that any possible energy source could keep up the acceleration for that long decreases.

[PM 2Ring]

please, please tell me that's a real module.

It could be, if you write it. But really, those equations gmalivuk linked to are pretty straightforward, so it's probably not worth the effort to make a new class for them. However, if you do write a relativity app in Python, please post it somewhere on these forums.

FWIW, that article comes from the old Usenet physics FAQ. I was a bit upset to see that it gives no credit. From http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html

Updated by Don Koks 2006.
Fuel numbers added by Don Koks 2004.
Updated by Phil Gibbs 1998.
Thanks to Bill Woods for correcting the fuel equation.
Original by Philip Gibbs 1996.

[Tass]

If you were capable of sustaining the 1g indefinitely, then IIRC 21 years will get you to the edge of the visible universe.

Of course by then billions of years will have passed in the rest of the universe, and you will contain enough kinetic energy to vaporize any planet you hit (or close, could someone do the math?).

[Ended]

If you were capable of sustaining the 1g indefinitely, then IIRC 21 years will get you to the edge of the visible universe.

Of course by then billions of years will have passed in the rest of the universe, and you will contain enough kinetic energy to vaporize any planet you hit

The OP might be interested in Tau Zero by Poul Anderson, which has basically this as the premise and explores some of the effects of travelling close to c.

[Ingolifs]

please, please tell me that's a real module.

It could be, if you write it. But really, those equations gmalivuk linked to are pretty straightforward, so it's probably not worth the effort to make a new class for them. However, if you do write a relativity app in Python, please post it somewhere on these forums.

FWIW, that article comes from the old Usenet physics FAQ. I was a bit upset to see that it gives no credit. From http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html

Fascinating read. I've always wanted to know how long it would take if you accelerate for half of the trip, turn around and decelerate for the other half. The bit at the end about the cosmic background radiation being blueshifted to the point where it would vapourise the craft at high enough velocities was especially interesting.

Of course, the problem with looking at this whole thing from a constant acceleration perspective is that it's not realistic. Because you'll be losing fuel mass, it would be more pertinent to look at a constant force and a decreasing mass. In this scenario, i'd imagine most of the trip time would be accelerating at a comparatively slow rate towards the destination. Only a small amount of time would be spent at the end decelerating, because the much lighter craft would experience a much greater deceleration from the same force. In this case, i'd imagine the maths would be a bit trickier to figure out to find the optimum time for acceleration and deceleration.

[skeptical scientist]

Not in a ramjet, which is probably the most feasible way of constantly accelerating/decelerating at 1g across an entire galaxy.

[gmalivuk]

Because you'll be losing fuel mass, it would be more pertinent to look at a constant force and a decreasing mass.

No, because you've still got to think of the passengers. Even though the thing at the end might be small enough to decelerate at 10g instead of 1, that's not a good idea if you want anyone to live through it.

[Vanzetti]

Because you'll be losing fuel mass, it would be more pertinent to look at a constant force and a decreasing mass.

No, because you've still got to think of the passengers. Even though the thing at the end might be small enough to decelerate at 10g instead of 1, that's not a good idea if you want anyone to live through it.

At the point where we can build Bussard Scramjet, I`m sure you will already have the technology to preserve passengers at high accelarations...

[nbonaparte]

Because you'll be losing fuel mass, it would be more pertinent to look at a constant force and a decreasing mass.

No, because you've still got to think of the passengers. Even though the thing at the end might be small enough to decelerate at 10g instead of 1, that's not a good idea if you want anyone to live through it.

At the point where we can build Bussard Scramjet, I`m sure you will already have the technology to preserve passengers at high accelarations...

In a singularity-esque world, you might not even have a physical body. You could decelerate at the speed your computer can take.

[gmalivuk]

At the point where we can build Bussard Scramjet, I`m sure you will already have the technology to preserve passengers at high accelarations...

Not necessarily. It's my understanding that high-g propulsion is mostly an engineering problem, so to speak, which could potentially be resolved with technological advances in today's theoretical framework. But if we still have our physical bodies, there is a limit to how much acceleration they can take, and I don't know that there is any way within current theory that can negate that fact.

[EdgarJPublius]

Not one mention of the glorious ProjectRho? For shame! Espescially since it has all the formulas and tables you need all in one convenient page

http://www.projectrho.com/rocket/rocket ... relativity

[Ingolifs]

Because you'll be losing fuel mass, it would be more pertinent to look at a constant force and a decreasing mass.

No, because you've still got to think of the passengers. Even though the thing at the end might be small enough to decelerate at 10g instead of 1, that's not a good idea if you want anyone to live through it.

You're making the assumption that there are humans on the ship. That isn't necessarily the case; the ship may only be fitted with instruments, or will be inhabited by some form of posthuman or AI designed with the rigours of space-travel in mind. Yes there is an upper limit on acceleration, but it's not as severe as you seem to think.

[nbonaparte]

Not one mention of the glorious ProjectRho? For shame! Espescially since it has all the formulas and tables you need all in one convenient page

http://www.projectrho.com/rocket/rocket ... relativity

Thank you sir. I just lost a great number of hours.

[Goemon]

Immersion in fluid would help reduce the effects of high g's, but doesn't eliminate them altogether. Some portions of your body (bones, teeth, muscle) are denser than other bits (blood, spleen, liver...). A good hard bump might just leave your skeleton at one end of the tank and the rest of you at the other end. And what's to stop your brain from sloshing to one side of your skull during those 500g turns? I don't think extra pressure on the outside would prevent that.

[PM 2Ring]

In Greg Bear's novel "Anvil of Stars" (the sequel to "Forge of God"), the protagonists have a spacecraft that uses intelligent "force field" technology to accelerate the ship's contents almost simultaneously, which allows very high accelerations. One problem with this approach is that the higher the acceleration, the smaller the individually accelerated "voxels" need to be, and if they're too small, this interferes with the proper functioning of the ships mechanical parts and the metabolic processes of the crew. Of course, in a novel, that's actually an advantage - it's hard to create good sci-fi gizmos that don't undermine the dramtic tension.

The other problem is that such force fields aren't very scientifically plausible.

Another more plausible technique is to pack the crew into some kind of gel for high-acceleration parts of the journey, which could allow much higher acceleration than water. Ideally, the gel is of high viscosity & low density. Oxygen permeability would be a bonus...

[kromagnon]

Oxygen permeability would be a bonus...

Wouldn't it actually be a necessity? I imagine you couldn't get too much of a benefit from floating in a gel unless your lungs were filled with it also.

[PM 2Ring]

Oxygen permeability would be a bonus...

Wouldn't it actually be a necessity? I imagine you couldn't get too much of a benefit from floating in a gel unless your lungs were filled with it also.

Well, your lungs won't be moving very much if they're full of gel & you're accelerating rapidly. I suspect you'd need to resort to intravenous methods to get fresh O₂ & eliminate CO₂. Any O₂ in the gel would just be to make it friendlier on the lung tissue.

[Vanzetti]

And what if the passengers are frozen?

[JWalker]

It is possible to negate the apparent force due to acceleration through a clever manipulation of the shape of spacetime via gravity. All one would have to do is produce a region of space in which the gravitational acceleration relative to an outside observer is equal to the desired acceleration of the ship. Observers in a ship in the gravitational field would think they were falling freely and feel no force, while according to the outside observer they would be accelerating. This is completely allowable in current physics, though it presents a large engineering challenge.

[thoughtfully]

Good Luck With That. (tm)

[JWalker]

Good Luck With That. (tm)

Hah, well, it would require a lot of energy, but the point is that it is possible with current physics. One method could be to generate a gravitational soliton that propagates at a speed less than the speed of light. If I remember right, such solutions to Einstein's field equations do exist; it would just be a matter of producing one and then riding it however far you wish.

[Tass]

And what if the passengers are frozen?

If the passengers are frozen, or we are only moving electronics or so, then there is no need to go fast. Just use fission reactors and ion drive to go .01c.